1. Field of the Invention
The present invention relates in general to pipe valves and more particularly to a ceramic rotary valve for use in a piping system which is required to have a good heat and corrosion resistance or a ceramic rotary valve used for accurately controlling the flow of fluid in a piping system.
2. Description of the Prior Art
An example of a prior art rotary valve for regulating the flow of fluid in a piping system is shown in FIG. 3. Referring to the figure, a rotary valve (A) is opened, i.e., its through hole (B) is aligned with a fluid passage (D) of a pipe (C) when the rotary valve (A) assumes the illustrated position through rotation about its center axis in the direction indicated by the arrow, thus allowing fluid to flow through the rotary valve (A). The rotary valve (A) is closed when further rotated in the same direction in case of a one way rotational type or rotated in the opposite direction in case of a two way rotational type. The rotary valve (A) is made of ceramics when fluid has a high temperature or fluid is highly corrosive.
In an automatic control or remote control of the flow of fluid in a piping system, it is necessary to accurately control opening and closing of the rotary valve (A). Particularly, in case where fluid to be controlled is gas, it is necessary to accurately control the rotary valve (A) with respect to the position where it starts opening and the position where it finishes closing.
However, when the rotatary valve (A) is made of ceramics, its baked outer periphery is poor in dimensional accuracy due to contraction at baking or firing. Further, since compacts differ in coefficient of contraction from each other, the angle of rotation of the rotary valve (A) between the position where it starts opening and the position where it finishes closing varies from product to product due to variations of coefficient of contraction. For this reason, it has been necessary to apply a complicated and delicate machining to the rotary valve (A) to attain the accurate dimensions.
In order to solve this problem, it has been proposed to form the through hole smaller than a finishing diameter at baking and then grind it to the required diameter. However, this requires a costly machining.